It is known for hand-held motor driven power tools, particularly screwdrivers, to incorporate a clutch overload mechanism, usually in the gearbox. The clutch is arranged to interrupt or break the drive train when a torque force applied to the power tool's output exceeds a threshold value. This can be achieved by causing components of the gear train to slip or ratchet with respect to one another. In this instance the motor continues to operate but the gearbox output, and hence the tool bit, does not rotate. Thus, the clutch can be used to prevent a nut or screw from being tightened beyond a certain torque (at which the thread might be stripped, for instance).
The gear train in conventional power tools usually has two or more gear reductions, and often incorporates a speed change facility. The gears are typically epicyclical, or planetary-type gears which provide relatively high reduction ratios for a compact size or volume. Such a gearbox for a power tool is described in EP0613758A1.
Conventional motor powered screwdrivers have the clutch arranged on the output gear of the gear train. Overload clutches are often of the ball-clutch type where a ball sits in a socket on a gear ring, as exemplified in EP0613758A1. The ball is urged into the socket by a load or force applied by a spring. The spring force can be varied by the user by adjusting a torque adjustment collar disposed around the gearbox output between the gearbox and chuck. Adjustment of the collar changes the compression of the spring, and hence the force applied by the spring to the ball-clutch. The torque required to cause the clutch to slip varies according to the spring's compression and/or the position of the collar. A clutch may employ pins, rather than balls, as described in EP1445074A1.
Disposing the clutch on the output gear of a reduction gear train (for instance, the third gear in a three gear train) results in a relatively high torque force being required before the clutch slips. This in turn requires a relatively large force applied to the clutch mechanism in order to maintain the clutch parts from slipping. As a result, a relatively large and heavy spring is required to apply the necessary forces.
To reduce the spring's size and weight, the clutch can be arranged on different parts of the gear train, where a lower torque force is required. For instance, the clutch can be arranged on a gear closer to the motor drive for a reduction gear train. In this arrangement, for conventional motor driven screwdrivers, the clutch adjustment collar (which the user sets the torque force at which the clutch ratchets) and spring are arranged around the gearbox output, extending from the chuck-end of the gearbox and adding to the length of the power tool. A transfer mechanism is required to apply the spring load to the clutch mechanism. The transfer mechanism is arranged to apply the load either through the gears, or around the gears. Such a transfer mechanism usually comprises link-pins or the like to couple the spring to the clutch plates. As a result, the weight saving achieved by reducing the spring size is minimised by the increased weight caused by the transfer mechanism.
EP302229A2 describes a clutch mechanism disposed on a third planetary gear. A range of torque can be set by adjusting a torque setting knob which adjusts the biasing force of a spring. The spring urges balls into recesses on the third gear. When the torque exceeds a load the third gear ratchets over the balls. Axial movement of the gear causes backward movement of slide pins which are connected to a gear of the first planetary gear. The pins act to push a brake disk, which normally stops the movement of the first gear, thereby allowing free movement of the first gear when the clutch ratchets.
In multi-speed multi-gear reduction gearboxes, there are problems associated with a clutch mechanism which is arranged on a gear after (or down stream of) a speed-change mechanism. The problem is that the torque clutch has a limited range over each speed. This is so because at a high speed setting (for a reduction gearbox) only some, and not all of the gear reductions are used. Thus, the output torque is limited to the motor's torque multiplied by the operating gears' reduction ratios. By comparison, when operating in the lowest speed, all the gear reductions are used and thus the output torque equals the motor's torque multiplied by all the gears' reduction ratios. As a result, a full range of torque can be applied by the output in low speed, but that range is not available in high speed. Thus, if the torque overload clutch is designed to ratchet at a maximum torque value which falls between the maximum torque output for the two speeds, then all the torque is available at low speed, but only a portion of the torque is available at high speed.
The present invention aims to ameliorate the problems with the prior art, some of which are discussed above.